1. Field of the Invention
This invention relates to electrophotographic transfer-fusing apparatus and more particularly to an improved electrophotographic toner transfer-fusing apparatus of the thermomechanical type.
2. Description of the Prior Art
In customary electrophotographic processes, a conductive backing having a photoconductive insulating layer thereon is electrostatically imaged by first uniformly charging its surface, and subsequently exposing the charged surface to a pattern of activating electromagnetic radiation, such as light. The radiation pattern selectively dissipates electrostatic charges in the illuminated areas on the photoconductive surface, which results in a latent electrostatic image in non-illuminated areas. This latent electrostatic image can be subsequently developed to form a visible image by depositing developer materials thereon by a variety of development techniques, the most common of which is cascade development.
Solid electrophotographic developer materials are often two-component systems containing finely divided pigmented particles, commonly called "toner," and relatively coarser, larger beads, commonly called "carrier beads." Toner particles and carrier beads are chosen or modified to make them triboelectrically dissimilar, so that triboelectric charges are generated upon each as they are transported through the development apparatus. Additionally, the components are chosen so that the toner will have triboelectric charges thereon opposite in sign to the electrostatic charge pattern to be developed. Thus, toner particles adhere to the carrier particles because of their electrostatic charges and because of van der Waals forces, until they are shaken loose during development and captured by the opposite and stronger electrostatic charges on the insulating surface to be developed.
Developed toner images can be fused to the photo-conductive insulating surface, or transferred and fused to another substrate such as plain paper. Transfer of the toner image can be accomplished by electrostatic methods, by the use of adhesive-coated paper, by pressure contact, etc. Once transferred, the toner image can be fused or fixed to the paper by heating and cooling, solvent vapor fusing, applying a fixative coating over the image, etc. The general state of the art in transfer and fusing methods and apparatus is described in more detail in Schaffert, Electrophotography, Focal Press, New York, (1965) at pages 24-25 and 39-40; and in the following U.S. patents: Lorenz, U.S. Pat No. 3,640,749; Welkel, U.S. Pat. No. 3,647,292; Colt et al., U.S. Pat. No. 3,647,499; Young, U.S. Pat. No. 3,640,249; Eastman, U.S. Pat. 3,630,691; Tyler, U.S. Pat. No. 3,612,685; Langdon, U.S. Pat. No. 3,612,677; Vince, U.S. Pat. No. 3,584,195; Schluntz, U.S. Pat. 3,558,853; White et al., U.S. Pat. No. 3,567,484; Fantuzzo, U.S. Pat. No. 3,566,076; and Byrne, U.S. Pat. No. 3,539,161.
In some cases, toner images have been transferred to intermediate belts prior to fusing on copy paper. Carlson, U.S. Pat. No. 2,990,278, for example, teaches the use of an intermediate transfer belt which has a smooth surface and which does not form a mechanical bond with toner. Carlson, U.S. Pat. No. 3,374,769 teaches the use of a transparent intermediate belt with a reflecting surface on the side of the belt opposite to a radiant heater. The transfer and fusing system described herein, while using an intermediate belt in some embodiments, has significant advantages over these prior art systems including: use of the belt as part of the paper transport system; use of the copy medium to cool the belt after each transfer; use of short, hot, selective heating zones; each of which features is described in considerably more detail infra.
One of the more recent efforts to improve pressure transfer and fusing techniques is described in Byrne, U.S. Pat. No. 3,591,276. In the method described therein, a developed xerographic powder image is transferred from a photoconductive surface to a final support material such as paper by contacting the toner image on the photoconductor with an elastomeric material under pressure to encapture the toner image due to the deformation of the elastomeric member. Subsequently, the toner image is transferred from the elastomeric member onto paper, either preheated or heated simultaneously by bringing the paper into pressure contact with the elastomeric member. Sufficient heat is supplied to the paper prior to or during pressure contact to cause the toner particles to melt and coalesce onto the paper.
While the techniques described by Byrne have improved pressure transfer and fusing, significant problems still remain. For example, since the paper is required to be significantly heated prior to or during transfer, high amounts of power are required especially since the paper normally contains moisture which must be driven off. Thus, high amounts of heat have to be supplied because of the use of an inefficient method of heating the toner. This results in a corresponding loss of economy and also causes certain other undesirable limitations for copy processes. Additionally, there can be a loss in the quality of fusing obtainable because of certain inherent limitations on the combination of heat and pressure which can be applied when preheated paper is depended upon to heat the toner to its melting point. In general, these inherent limitation arise because the paper can be heated only so high and because steep pressure pulses are not used at the transfer point.
Another recent effort to improve pressure transfer and fusing techniques is described in Sanders et al., U.S. Pat. No. 3,669,706. The fusing device described therein has a radiant energy transmissive rotatable drum covered by a radiant energy transmissive resiliently compressible layer and a thin flexible radiant energy absorbing outer skin. A major drawback to this system is its inefficient thermal design. For examle, the compressible layers are purposely designed to absorb large amounts of heat since the inner layer is relatively thick and since the outer layer is radiation absorbing. Also, thermal inefficiency occurs since a drum substrate is required between the heat source and toner, and because heat is applied to the bottom side of the toner. Since the drum heats up, the concomitant disadvantage of lessening the life of the photoconductor is also present. Higher pressure at the transfer point is also required, since heat passes from the drum surface to the toner by conductive heating rather than radiation heating; thick layers make it difficult, however, to achieve a pressure pulse with a fast rise time. Additionally, since the elastomer is relatively thick, dimensional distortion of the image is increased.
It is desirable, therefore, to have toner transfer and fusing methods and apparatus which can take advantage of the improvements offered by Byrne and Sanders et al. while not suffering from their inherent disadvantages.